The object of the invention is to provide a polymer blend with excellent transparency and thermal resistance. Applicants have found that incorporating tribromophenyl maleimide comonomer into a methyl methacrylate polymer provides for methyl methacrylate-tribromophenyl maleimide copolymers which exhibit enhanced miscibility with polycarbonate, such that melt-processible blends of polycarbonate having excellent transparency and thermal resistance over the entire range of blend compositions (from 99% of methacrylate copolymer to 99% of polycarbonate) are possible.
Polymer blends are very important to the plastic industry and are well known. Miscible polymer blends, often referred to as polymer alloys, are desirable and useful because they provide a unique combination of properties from two or more polymers, that one polymer alone cannot provide. In particular, blend polymers such as polymethyl methacrylate (pMMA) and polycarbonate, for example, bisphenol-A polycarbonate (PC) would be highly desirable and commercially useful materials for producing miscible transparent polymer blends. These materials are desirable and useful because pMMA is a clear, relatively inexpensive polymer with excellent light transmission and resistance to sunlight and PC is a clear polymer, with the advantageous properties of toughness and high heat resistance. Unfortunately, pMMA lacks heat resistance and is brittle, so it cannot be used in all applications and PC is a relatively expensive polymer. It is therefore desirable to achieve fully miscible polymer blends of pMMA and PC. Such blends would offer variable property/cost behavior properties based on blend ratios of pMMA and PC between the limits of pure pMMA and pure PC, while maintaining optical clarity.
Unfortunately, pMMA and PC are known not to be compatible or miscible. Melt processed blends of pMMA and PC having commercially useful molecular weights, such as weight average molecular weights of about 70,000 or more, are not homogeneous. Phase separation of such blends occurs at temperatures just above the glass transition temperature of the blend. The melt processing temperature required for polycarbonate or PC is typically 100 degrees Celsius (.degree.C.) above its glass transition temperature (Tg), and in this case about 100.degree. C. above the Tg range for polycarbonates, of 150.degree. C. to 250.degree. C., and at that processing temperature, blends of pMMA and PC are inhomogeneous and opaque, and have undesirable mechanical properties. It is therefore desirable to achieve a blend of pMMA and PC that remains as a single phase at melt-processing temperatures.
The "cloud point temperature" is an important factor in blend miscibility, as it is the temperature at which the blend changes from clear to "cloudy," because of phase separation. To be melt-processible, the cloud point of the PC and MMA copolymer blend should be .gtoreq.250.degree. C. If a comonomer is not particularly efficient at altering the phase behavior of the blend, a large amount of the comonomer must be added, thus raising the expense of the copolymer and/or changing the physical properties of methyl methacrylate polymer.
Examples whereby the phase behavior of PC and pMMA has been altered by chemical modification exist in the literature. Examples of known art are discussed below as are the deficiencies of this known art and hence illustrate the advantages of the present invention.
In the closest known reference, U.S. Pat. No. 4,491,647 ('647) discloses a partially miscible blend or a polyblend of polycarbonate and N-phenylmaleimidemethyl methacrylate copolymer. These partially miscible blends are taught as ternary compatibilizers for polycarbonates with acrylonitrile-butadiene-styrene (ABS) resins or rubber-modified styrene-maleic anhydride resins.
Additional references teach comonomers polymerized with methyl methacrylate 25 to improve the thermal stability, but which hinder miscibility with PC. For instance, U.S. Pat. No. 5,073,615 and JP 61-162,509 both teach the copolymerization of methyl methacrylate with various N-substituted maleimide compounds, including methyl, ethyl, phenyl and cyclohexyl maleimide. Many of these maleimides, however, render the methacrylate copolymer incompatible with PC resin, particularly phenyl maleimide (PMI), as discussed earlier in '647. While cyclohexyl maleimide enhances the miscibility with PC, it is not an efficient comonomer in altering the phase behavior, and as such does not raise the cloud point sufficiently that all blend ratios are melt-processible. U.S. Pat. No. 4,950,716 demonstrates the copolymerization of cyclohexyl maleimide and the like with methyl methacrylate, but shows that the cloud points of the blends were not sufficiently raised such that some blend compositions were opaque after melt-processing.
EP 483,717 teaches copolymerization of methyl methacrylate with ring-halogenated phenyl methacrylate, for example tribromophenyl methacrylate. While the tribromophenyl methacrylate comonomer appears to be somewhat efficient at raising the cloud point temperature of the blend, relatively large amounts of the comonomer need to be incorporated into the copolymer, for instance .gtoreq.20 weight percent (wt. %). Further, tribromophenyl methacrylate has the ability to homopolymerize, and its incorporation into a copolymer with methyl methacrylate is not straightforward. Special synthetic techniques are required to prevent homopolymerization of the comonomer, which would result in formation of an opaque copolymer.
Therefore, in all known references, there are deficiencies with the approaches used to achieve fully miscible blends of pMMA and PC that can be melt-processed. Deficiencies include the molecular weights of the polymers must be reduced, or the comonomer added to methyl methacrylate is not efficient or is difficult to polymerize or adds great expense, or comonomer, such as methacrylate based esters undesirably change the properties of the methacrylate copolymer especially with regard to thermal stability. In the case where termonomers are needed to enhance thermal stability, often miscibility with PC is diminished or expense is added for the manufacture of the copolymer. Applicants have overcome these deficiencies with this invention.